In order to stably manufacture high flatness semiconductor wafers, it is necessary to polish semiconductor wafers so as to have a target final thickness.
In conventional polishing methods, the polishing time of a current polishing batch is calculated on the basis of a polishing speed in a previous polishing batch, e.g. in the start of operation, to finish a wafer with a target thickness.
In this method, however, changes of a polishing state, e.g. due to the wear of a carrier, a polishing pad, and a polishing slurry cause a difference between an actual polishing rate and a calculated polishing rate. This makes it difficult to obtain the target final thickness in every batch.
Such a deviation of a final thickness from the target final thickness in polishing is one of reasons for deterioration of flatness.
It is therefore necessary to polish the semiconductor wafer while the final thickness of the wafer is detected. A device for measuring a thickness is called as a sizing device.
As examples of the sizing device, there exist an optical-type device that directly measures the wafer thickness, an eddy-current-type device, an electrostatic-capacity-type device, and a device having a crystal plate that measures the wafer thickness by resonance, i.e. a transit method-type device (e.g. See Patent Document 1).
For example, when the thickness is measured with a sensor of the type that has a narrow measuring range, such as the eddy-current sensor and the electrostatic capacity sensor, the sensor needs to approach the wafer in measurement. In a conventional double-side polishing apparatus as shown in FIG. 3, accordingly, a through-hole 108 is provided at an upper turn table 102 in the direction of an upper-turn-table rotation axis, and the sensor is disposed near the lower end in the through-hole 108 of the upper turn table 102, i.e. at a position near the wafer.
In this case, a sensor holder 107 is needed and the sensor 106 is held at the end (the lower end) of the sensor holder 107.
For example, the sensor holder 107 is slightly smaller than the through-hole 108 provided at the upper turn table 102 so as not to make direct contact with the upper turn table 102. The sensor holder 107 is fixed at the upper portion of the upper turn table. The sensor 106 is fixed so as to locate at approximately 500 μm away from a polishing pad 104. The interior of the sensor holder 107 is hollowed to reduce heat conduction. The sensor holder is made of metal material such as super invar, and hung from the upper surface of the upper turn table 102 to be attached.
Double-side polishing of the wafer is performed while the wafer thickness is measured by using the sensor held with the sensor holder to finish the wafer with the target thickness.